1. Field of the Invention
The present invention relates to a data-track search apparatus for searching for a desired track on a record carrier having a great number of tracks.
2. Description of Prior Art
There exists as a data recording apparatus an optical reproducing apparatus which focuses and radiates a light beam emitted from a light source such as a semiconductor laser diode on a disk-like record carrier rotating at a constant rotation speed.
An example of such an apparatus is the optical apparatus as disclosed in "High speed accessing magneto-optical disk drive" (SPIE Vol. 899-Optical Storage Technology and Applications. 1988) by H. Ishibashi et al.
Provided on the record carrier of this apparatus are spirally or concentrically minute tracks having a width of 0.6 .mu.m and a pitch of 1.6 .mu.m. When signals recorded on the record carrier are reproduced, tracking is so controlled as to enable the light beam to be positioned on the track at all times, and a photodetector acts to receive a reflected light from the record carrier. A tracking error signal is similarly detected by the reflected light from the record carrier for effecting a tracking control so that the light beam will be put in position on the track of the record carrier. The tracking control is performed by feeding this tracking error signal back to a tracking actuator for moving the light beam incident on the record carrier in the direction traversing the tracks.
The function of search is indispensable for positioning the light beam on a desired one of many tracks contained on the record carrier.
The search is carried out by driving the tracking actuator in the direction toward of the desired track in such a manner that the light beam will go across the tracks with the tracking control out of action and restarting the tracking control on the desired track. For obtaining a speedy and stable search the tracking actuator is speed controlled so that the light beam may move at an almost determined velocity during the search. The speed control needs information of the moving speed and position of the light. The moving speed and position of the light beam is detected by a tracking error signal which may be generated when the light beam crosses the tracks as it is urged by the tracking actuator.
The speed of the light beam is detected by the tracking error signal cycle. The position of the light beam is detected by detecting from the tracking error signal a track crossing signal of alerting when the light beam has crossed the center of the tracks and counting the track crossing signal from the search start track.
In the conventional method, however, if a failure of the focusing control takes place due to vibrations or impact during search, the tracking error signal will be a constant value in spite of the fact that the tracking actuator is in motion. This, thus, makes it impossible to detect the moving speed of the light beam, and the driving amount of the tracking actuator will be correspond to the just previous moving speed. The impossibility of detecting the number of the tracks crossed will also result in the failure to detect the desired track. The tracking actuator will keep moving, accordingly, until it runs against the leader or termination. If the focusing control failure occurs when the tracking actuator drives to a great extent as it accelerates at the start of search, it will maintain its great driving amount. In such a case, the tracking actuator may run at an exorbitantly high speed against the leader or termination, and it may destroy itself in some cases.